The present invention relates to a process for the continuous dry granulation of powdered carbon black.
When processing industrial carbon black, granulated products are preferably used, these often being called carbon black granules, carbon black beads, pelleted or pelletized carbon black.
The individual granules or beads of these carbon black products are spherical. Depending on the particular application, a mean bead diameter d.sub.50 of from 0.125 to 2.0 mm is desired. Carbon black beads with mean diameters of less than 0.125 mm are referred to in the art as fines and are generally considered undesirable. When using granulated carbon black as a filler for rubber blends and as a pigment for plastics or dyes, the granulated carbon black is substantially well dispersed in the ultimate product. Since the bead hardness and thus also the dispersion hardness in a given granulating process, from experience, depends on the diameter of the carbon black beads, carbon black granular materials with as narrow as possible a bead size distribution curve are desired. Bead size distributions with a ratio d.sub.80 /d.sub.20 of less then 9 are desirable.
Two different processes are currently used for granulating carbon black on an industrial scale, namely, wet granulation in a beading machine with a subsequent drying procedure and dry granulation in a pelletizing drum. The two processes have quite different process parameters which are closely related to the physical process involved in the particular agglomeration procedure and the resulting pellet properties.
Granulators with toothed shafts are used as pelletizing machines for wet granulation. In the following, the expressions granulator and toothed shaft granulator are used as alternatives to pelletizing machine. Toothed shaft granulators consist of a fixed drum arranged in a horizontal position (also called a stator herein) with a rotating toothed shaft located therein.
The teeth are pins of a specific diameter and a specific length which are arranged on the toothed shaft, axially offset with respect to each other, along one or more helical lines. The pins are aligned in a radial direction. Their length is such that the distance from the ends of the pins to the internal wall of the stator is normally 2 to 10 mm.
A pelletizing space is provided for granulation between the axis of the toothed shaft and the stator wall. In the pelletizing space the carbon black is transported by the rotating toothed shaft from the inlet at one end of the stator to the exit at the other end of the stator. Agglomeration then takes place due to the carbon black rolling along the stationary stator wall. The residence time of the carbon black in the mixer granulator can be extended by fitting a baffle plate at the exit or by raising the exit with respect to the inlet. Typical pelletizing machines have a length of 1 to 3.5 m and a diameter between 200 and 760 mm.
Inside the pelletizing machine, the powdered industrial carbon black is intensively mixed with water, optionally with the addition of a binder. Depending on the type of carbon black, at water contents of from 40 to 60 wt. %, with reference to the total weight, spherical carbon black pellets are obtained. The moist pellets are then dried in a further process step (DE-AS 1 264 412; U.S. Pat. Nos. 3,607,086; 3,787,161 and 4,222,727).
During wet granulation agglomeration takes place by means of liquid bridges and capillary forces between the carbon black particles. The size of the capillary forces enables comparatively high peripheral speeds of 10 to 20 m/s at the ends of the teeth, wherein intensive blending and subsequent granulation are produced.
The residence time required for effective formation of carbon black pellets is in the region of a few seconds, normally less than 20 seconds, with wet granulation. The residence time can be extended to the region of a few minutes by means of a baffle plate or by raising the exit with respect to the inlet.
The bead hardness which can be produced for carbon black pellets obtained by wet granulation, without using binders, is in the range from 0.1 to 0.3 N for pellet diameters from 1.4 to 1.7 mm. The average bead size (d.sub.50 value) can be varied from 0.2 to 2.0 mm during wet granulation by appropriate adjustment of the process parameters.
In the case of dry granulation, pelletizing drums are used which also consist of a horizontally arranged rotating tube. The internal space of the tube is called the pelletizing chamber. In order to achieve granulation, powdered industrial carbon black is precompacted in accordance with DE 38 32 404 and granulated in the pelletizing drum by rolling round on the rotating tubular wall. The van der Waals and electrostatic forces responsible for dry granulation are substantially smaller than the capillary forces which act during wet granulation. Accordingly, granulation must be performed more gently. The peripheral speed of the pelletizing drum is therefore only 1 to 2 m/s. If the peripheral speed is much higher than this, the rolling motion does not occur due to the high centrifugal force. Furthermore the effect of the force on the pellets being formed is so large that these immediately disintegrate again. The pellet hardness which can be produced is usually less than 0.1 N for pellet diameters of 1.4 to 1.7 mm. The average pellet size (d.sub.50 value) with conventional dry granulation is generally in the range from 0.125 to 0.8 mm.
Due to the small van der Waals forces, to initiate granulation it is necessary to add seeding material, in the form of prepelleted carbon black, to the carbon black. The seeding material is added to the pelletizing drum, before the start or continuously during operation of the pelletizing drum, in an amount of 1 to 30 wt. %, with reference to the amount of powdered carbon black. The seeding material ensures that the seeds required for agglomeration are always available in adequate amounts. If seeding material is not added, dry granulation may not take place or the pellets may break up.
The typical residence time for carbon black in a dry pelletizing drum is 1 to 4 hours. In order to ensure sufficient carbon black throughput (in kg/h), dry pelletizing drums have to be substantially larger than pelletizing machines for wet granulation. In production, dry pelletizing drums with diameters of 2 m and lengths of 18 m are used. The filling amount used in these types of drums is several tons. In contrast, the amount of material used in pelletizing machines is only a few kilograms.
Additives to increase the bead hardness and/or to improve dispersibility may be used during both wet and dry granulation.
Wet granulated carbon blacks generally also have a higher dispersion hardness than dry granulated carbon blacks due to their higher bead hardness. They are therefore mainly used in the rubber industry. Wet granulated carbon blacks can be efficiently dispersed in highly viscous rubber materials. Their high bead hardness means that the mixture is easy to transport in pneumatic feeding units.
Dry granulated carbon blacks are mainly used as pigments in lacquers, printing inks and plastic materials. Powdered carbon black is also still used in large amounts in these fields of application, however, and this can lead to extensive dust pollution in the workplace during handling procedures. This dust pollution can be reduced only by using granulated carbon blacks. It is expected that the demand for granulated carbon blacks will increase in the future. This requires considerable investment from carbon black producers due to the size of the plant.
An object of the present invention is to provide a process for dry granulation which facilitates dry granulation with the same throughput as in known dry pelletizing drums in substantially smaller plants or increases the throughput of existing plants and provides a carbon black granular material which is distinguished by a narrow distribution curve for pellet diameters, good dispersibility and the smallest possible amount of fines (pellet diameter &lt;0.125 mm).